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How Do Reverse Osmosis Filter Systems Work & What Do They Do?

These are questions which we have been asked numerous times when helping our customers select a filters or a water filtration system which is right for them.
Below you will find diagrams and answers to those questions. If you have other questions which are not answered, please visit our FAQ page or Contact Us.

Producing Drinking Water Using Reverse Osmosis

Although Reverse Osmosis seems like a complex system it is really a simple and straightforward water filtration process. And it's not a new process. High-pressure (pump driven) reverse osmosis systems have been used for years to desalinate* water – to convert brackish or seawater to drinking water. Having a better understanding of how a reverse osmosis system works will eliminate the mystery and confusion you may feel when you look at a reverse osmosis system -- with its many colored tubes and multitude of filters. Read on to enhance your knowledge of residential reverse osmosis systems.

The most important points to remember:

  • All RO Systems work the same way.
  • Most RO (Reverse Osmosis) systems look alike.
  • All RO Systems have the same basic components.
  • The real difference is the quality of the filters and membranes inside the RO.

reverse osmosis diagram


How the Reverse Osmosis System Works?

Reverse Osmosis is a process in which dissolved inorganic solids (such as salts) are removed from a solution (such as water). This is accomplished by household water pressure pushing the tap water through a semi permeable membrane. The membrane (which is about as thick as cellophane) allows only the water to pass through, not the impurities or contaminates. These impurities and contaminates are flushed down the drain.

For a definition of **Reverse Osmosis.

Ultimately, the factors that affect the performance of a Reverse Osmosis System are:

  • Incoming water pressure
  • Water Temperature
  • Type and number of total dissolved solids (TDS) in the tap water
  • The quality of the filters and membranes used in the RO System (see operating specs)

Diagram of a Reverse Osmosis Membrane:

Reverse Osmosis Membrane Diagram


What does a Reverse Osmosis System Remove?

A reverse osmosis membrane will remove impurities and particles larger than .001 microns.

Reverse Osmosis System Removals


Elements and the Percent R.O. Membranes will remove


85 - 94%
96 - 98%
94 - 98%
85 - 95%
60 –75%
94 – 98%
95 – 98%
95 – 98%
94 – 96%
96 – 98%
95 – 98%
92 – 96%
94 – 98%
96 – 98%
85 - 92%
94 – 98%
95 – 98%
95 – 98%
84 – 92%
85 – 92%

% may vary based on membrane type water pressure, temperature & TDS


Basic components common to all Reverse Osmosis Systems:

  1. Cold Water Line Valve:   Valve that fits onto the cold water supply line. The valve has a tube that attaches to the inlet side of the RO pre filter. This is the water source for the RO system.
  2. Pre-Filter (s):   Water from the cold water supply line enters the Reverse Osmosis Pre Filter first. There may be more than one pre-filter used in a Reverse Osmosis system. The most commonly used pre-filters are sediment filters. These are used to remove sand silt, dirt and other sediment. Additionally, carbon filters may be used to remove chlorine, which can have a negative effect on TFC (thin film composite) & TFM (thin film material) membranes. Carbon pre filters are not used if the RO system contains a CTA (cellulose tri-acetate) membrane.
  3. Reverse Osmosis Membrane:   The Reverse Osmosis Membrane is the heart of the system. The most commonly used is a spiral wound of which there are two options: the CTA (cellulose tri-acetate), which is chlorine tolerant, and the TFC/TFM (thin film composite/material), which is not chlorine tolerant.
  4. Post filter (s):   After the water leaves the RO storage tank, but before going to the RO faucet, the product water goes through the post filter (s). The post filter (s) is generally carbon (either in granular or carbon block form). Any remaining tastes and odors are removed from the product water by post filtration.
  5. Automatic Shut Off Valve (SOV):  To conserve water, the RO system has an automatic shutoff valve. When the storage tank is full (this may vary based upon the incoming water pressure) this valve stops any further water from entering the membrane, thereby stopping water production. By shutting off the flow this valve also stops water from flowing to the drain. Once water is drawn from the RO drinking water faucet, the pressure in the tank drops and the shut off valves opens, allowing water to flow to the membrane and waste-water (water containing contaminants) to flow down the drain.
  6. Check Valve:   A check valve is located in the outlet end of the RO membrane housing. The check valve prevents the backward flow or product water from the RO storage tank. A backward flow could rupture the RO membrane.
  7. Flow Restrictor:   Water flow through the RO membrane is regulated by a flow control. There are many different styles of flow controls. This device maintains the flow rate required to obtain the highest quality drinking water (based on the gallon capacity of the membrane). It also helps maintain pressure on the inlet side of the membrane. Without the flow control very little drinking water would be produced because all the incoming tap water would take the path of least resistance and simply flow down the drain line. The flow control is located in the RO drain line tubing.
  8. Storage Tank:   The standard RO storage tank holds up to 2.5 gallons of water. A bladder inside the tank keeps water pressurized in the tank when it is full.
  9. Faucet:   The RO unit uses its own faucet, which is usually installed on the kitchen sink. In areas where required by plumbing codes an air-gap faucet is generally used.
  10. Drain line:   This line runs from the outlet end of the Reverse Osmosis membrane housing to the drain. This line is used to dispose of the impurities and contaminants found in the incoming water source (tap water). The flow control is also installed in this line.


Diagram of a Reverse Osmosis System with Basic Components:

Reverse Osmosis System Diagram


Quality of RO Membranes and Filters – They're not all alike!

While one RO System may look just like the next in terms of design and components, the quality of those components can be very different. These differences can have a significant impact on the quality of the water the system produces.

Here are some examples of questions you might ask and consequences associated with "less than desirable" quality.

  • Has the manufacturer used sound methods? What types of welds have been used in these plastic products? Will they allow contaminated water to bypass the filtration system? Will they allow the system to leak?
  • How has this filter or membrane been created? Will it allow the water to 'channel' and, in effect, bypass the removal component of this device?
  • What about the quality of the 'fill'? Are it's contents of a high enough quality to produce the expected percentage of contaminant reduction? Carbon quality, for instance, can have huge variances in reduction capability, reduction capacity, and the sloughing of 'fines', which can prematurely clog or foul the RO Membrane.
  • What are the manufacturer's controls on tolerances or variations in specifications? If this component is rated as a 1-micron filter will it truly filter out everything larger than 1 micron or will it only do the job 80% of the time? And, what if it actually filters at a .5-micron rate? That will stop the system from flowing -- clogging it and forcing filter replacement? If this is a sediment filter and it fails the excess sediment will clog or foul the RO Membrane.
  • And in general - Are the materials used in this product FDA or NSF (National Safety Foundation) approved? If not, you might question their quality or performance ability.

So, it becomes clear that the quality of the components is the key to an optimal functioning RO System.

Why and How To Increase the Gallon Per Day Capacity of A Reverse Osmosis Systems


The main reason to change to a higher flow reverse osmosis membrane is to improve the recovery rate which is to reduce the amount of time it takes to refill the storage tank.  This insures that there is adequate water available during times of heavy usage or when the reverses osmosis system may feed more than one location such as an ice maker and a dispensing faucet.


Changing to a higher flow membrane has no effect on the quality of the water your reverse osmosis system makes or the length of time the reverse osmosis membrane will last.


The change to a higher capacity membrane is easy.  You simply replace your old membrane with a new, higher capacity membrane, along with the correctly sized drain line flow restrictor.  (Matching the membrane with the correctly sized drain line flow restrictor is important to ensure the proper product to waste ratio is meet.  A mis-matched combination will allow either excess water to flow to the drain or cause premature fouling of the membrane.)  Most standard reverse osmosis membrane housings will accommodate membranes ranging in capacities from 10 – 150 gallons per day. 

Flow Control Installation Instructions

A complete listing of standard residential reverses osmosis membranes and corresponding drain line flow controls is on our membrane page or review the flow control guideline chart for residential reverse osmosis membranes on our parts page.


Where Can I purchase a Quality Reverse Osmosis System or replacement filters and membranes?

We carry replacement filters and membranes for most major brands of Reverse Osmosis systems. View our list of Reverse Osmosis Filters by brands that we carry.

We also have new Reverse Osmosis Systems available to purchase. See the features and benefits of each type of Reverse Osmosis System.

*Desalinate - The removal of dissolved inorganic solids (salts) from a solution such as water to produce a liquid which is free from dissolved salts.

**Osmosis – The natural tendency for water molecules to pass through a semi permeable membrane, from the side low in dissolved impurities to the side high in dissolved impurities.

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